A static pressure type non-contact gas seal 101 as formed and shown in FIG. 8 is known.
As shown in FIG. 8, the prior art seal 101 comprises a rotary seal ring 102 fixed on a rotary shaft 110, a stationary seal ring 104 held slidable in the axial direction via a pair of O-rings 106 on the circular inner circumferential portion of a seal casing 103 and springs 105 mounted between the back side of the stationary seal ring 104 and the seal casing 103 for pressing the stationary seal ring 104 against the rotary seal ring 102. And this prior art seal is so designed that the seal end faces 120 and 140 formed on the opposing ends of the two seal rings 102 and 104 are held in a non-contact state by a static pressure acting therebetween such that a circular area between the seal end faces 120 and 140 seals off the sealed fluid region at the outer periphery side, i.e. the inside region F of the machine, from the outside (atmospheric) region A at the inner periphery side.
The stationary seal ring 104 is provided with shallow grooves or static pressure generating grooves 109 on the seal end face 140. Into these grooves are introduced a seal gas 108 such as nitrogen gas compressed to a higher level than the pressure of the inside region F of the machine to produce a static pressure at the circular area between seal end faces 120 and 140, thus holding the seal end faces 120 and 140 in a non-contact state. In other words, the seal gas 108 led into the static pressure generating grooves 109 forms a static pressure fluid film between the seal end faces 120 and 140. Because of the presence of this fluid film, the seal end faces 120 and 140 are held in a non-contact state, with the inside region F of the machine sealed off from the outside region A. The seal gas 108 whose pressure is higher than that of the inside region F can leak through between the seal end faces 120 and 140 into the inside region F of the machine and the outside region A, but the inside gas or sealed fluid in the inside region F can not penetrate into the area between the seal end faces 120 and 140, and, hence, is not allowed to leak out to the outside region A.
To the static pressure generating grooves 109, the seal gas 108 is supplied through a continuous gas supply channel 180 formed in the seal casing 103 and the stationary seal ring 104 as shown in FIG. 8. The gas supply channel 180 comprises a circular closed space 171 and two passages 181 and 182. The circular closed space 171 is formed between the inner circumferential portion of the seal casing 103 and the outer circumferential portion of the stationary seal ring 104 and closed by a pair of O-rings disposed side by side in the axial direction. The first passage 181 is formed in the seal casing 103 through which the seal gas 108 is led into the closed space 171. The second passage 182 is provided in the stationary seal ring 104. Through this passage 182, the seal gas 108 supplied to the closed space 171 is led to the static pressure generating grooves 109 via an orifice 183. The seal gas 108 introduced into the static pressure generating grooves 109 flows out to the inside region F and the outside region A through between the seal end faces 120 and 140, holding the seal end faces 120 and 140 in a noncontact state by a fluid film formed by a static pressure. Between the seal end faces 120 and 140, an opening force and a closing force are in equilibrium and balanced, thereby keeping the two seal end faces in a non-contact state. The opening force is a force resulting from the static pressure produced by the seal gas 108 led to the static pressure generating grooves 109 and the pressure inside the machine acting on the outside circumferential edge 148 of the stationary seal ring 104 (the circular face outside the seal end face 140 of the stationary seal ring 104). The closing force is produced by the spring 105 that thrusts the stationary seal ring 104 against the rotary seal ring 102. The pressure of the seal gas 108 which is led to the static pressure generating grooves 109 is regulated depending of the pressure inside the machine and set to a higher level than the latter. The spring force (spring load) applied by the spring 105 which determines the closing force is so adjusted according to the pressure of the seal gas 108 that the gap between the seal end faces 120 and 140 may be proper (generally 5 to 15 .mu.m). The seal gas is squeezed by the orifice 183 before being led to the static pressure generating grooves 109 such that when the gap between the seal end faces 120 and 140 changes, the gap will be automatically adjusted and maintained properly. In other words, if the gap between the seal end faces 120 and 140 is widened by, for instance, vibration of the rotary components, the amount of the seal gas flowing into between the seal end faces 120 and 140 from the static pressure generating grooves 109 and the amount of the seal gas supplied to the static pressure generating grooves 109 through the orifice 183 will be put out of balance. As the pressure decreases in the static pressure generating grooves, the opening force will be smaller than the closing force with the result that the gap between the seal end faces 120 and 140 is narrowed to a proper size. Conversely, if the gap between the seal end faces 120 and 140 becomes narrow, the pressure in the static pressure generating grooves 109 will rise through the aforesaid mechanism of the orifice with the result that the opening force exceeds the closing force for enlarging the gap between the seal end faces 120 and 140 to a proper amount.
The prior art seal 101, like the dynamic pressure type non-contact gas seal, can effectively seal the gas inside the machine for a long time with the seal end faces 120 and 140 maintained in a non-contact state and with no seizing inflicted on the faces 120 and 140. Furthermore, the prior art seal 101 is effective in sealing the kind of gas which cannot be sealed by the dynamic pressure-type non-contact gas seal and offers a greater possibility of application than the latter. That is to say, the dynamic pressure-type gas seal, as is known, has dynamic pressure generating grooves formed on one seal end face that rotates relatively. With operation of these dynamic pressure generating grooves, a dynamic pressure is produced between the seal end faces by the gas inside the machine so that the seal end faces are kept in a non-contact state. In principle, this seal tolerates leaking of the gas inside the machine to the outside through between the seal end faces. Therefore, the dynamic pressure-type non-contact gas seal cannot be used for the kind of gas which must not be leaked outside, including toxic gas, flammable gas, and explosive gas. On the other hand, the prior art static pressure-type non-contact seal 101 is so constructed that the seal gas 108 with a higher pressure than the pressure inside the machine is supplied to the area between the seal end faces 120 and 140, thus completely preventing leakage of the gas inside the machine to the outside, and can effectively work with rotary equipment employed with such gases as toxic gas, flammable gas, and explosive gas.
While the prior art seal 101 has an advantage over the dynamic pressure type non-contact gas seal, the former present problems as described below when operated at a high level of pressure inside the machine, and cannot work very well with rotary equipment operated at the high pressure.
With the prior art seal 101, the pressure of the seal gas 108 is set higher than the pressure inside the machine to keep the gas inside the machine from leaking out. Under high pressure operating conditions, the opening force would be extremely great. To counter the great opening force, the spring force of the spring 105 has to be set higher than when working under low pressure conditions so as to maintain the gap properly between the seal end faces 120 and 140 with the two forces balanced in equilibrium. On the other hand, when the operation is suspended with the supply of the seal gas 108 cut off, the opening force and the closing force will be put out of balance. With the stationary seal ring 104 pressed against the rotary seal ring by the spring 105, the gap between the seal end faces 120 and 140 will be closed. Hence, if the spring force of the spring 105 is great, the stationary seal ring 104 may violently crash against the rotary seal ring 102 when the supply of the seal gas 108 is cut off, which may damage the seal rings 102 and 104 or the seal end faces 120 and 140.
Another problem with the prior art seal 101 is that the closing force, which is produced by the spring load alone and inevitably stays constant, cannot adapt itself to change in the inside pressure. Hence, under the conditions that the pressure inside the machine is changeable, the prior art seal 101 cannot work as an effective seal and is not suitable for use in rotary equipment operated under such conditions.
That is to say, the prior art seal 101 is so designed that the seal end faces 120 and 140 are kept in a non-contact with the closing force and the opening force balanced in equilibrium, wherein the closing force is produced by the spring load and the opening force results from the pressure of the seal gas 108 led to the static pressure grooves 109 and the pressure inside the machine acting on the outside circumferential edge 148 of the stationary seal ring 104. In this regard, the spring load and the pressure of the seal gas 108 are fixed. If the pressure inside the machine changes and, as a result, the opening force changes in accordance with the changes in the pressure inside the machine, the opening force and the closing force will be put out of balance. As a result, the gap between the seal end faces 120 and 140 cannot be maintained properly, thereby failing to satisfactorily seal the gas inside the machine. If, for example, the inside pressure exceeds the designed level on the basis of which the seal gas pressure and the spring load have been set, the closing force will be insufficient, thereby allowing the seal end faces 120 and 140 to open more than necessary, which can cause the inside gas to leak out to the region A outside the machine. If, on the other hand, the inside pressure falls below the designed pressure level, the opening force will be insufficient, which may result in the seal end faces 120 and 140 coming in contact with each other.
Where the pressure inside the machine fluctuates, it may be possible that the pressure of the seal gas 108 is regulated and controlled according to the fluctuations in the pressure inside the machine. In the prior art seal 101, however, since the closing force depends on the spring load alone and is fixed, such method cannot be adopted. In other words, if the opening force is change through pressure control of the seal gas 108, the closing force will become too large or too small in relation to the opening force. In the end, that would produce the same problem as encountered with a fixed pressure of the seal gas 108.
As set forth above, the prior art seal 101 cannot work effectively under high pressure or pressure changing conditions, and its application possibility is quite limited.